Transfer pumps of the "gear within a gear" configuration are used in many applications to pump fluids at relatively high flow rates at relatively low pressures (less than 500 psi) as compared to hydraulic system pumps which often operate at pressures well in excess of 1000 psi. This type of transfer pump, usually includes an internal crescent positioned between an outer, driven gear (alternately termed a "rotor") and a smaller, idler gear. The outer gear is connected to a shaft that extends through the housing and is attached directly or indirectly to a drive motor. The idler gear rotates about a fixed idler pin and is driven by the outer gear, as distinguished from a "gerotor" type of gear pump in which an "outer gear" includes inwardly directed teeth that mesh with, and are driven by, an internal drive gear.
A gear within a gear type of transfer pump is called upon to perform a wide variety of tasks in a wide variety of environments. Due to the configuration of the pump, the outer gear or rotor is subjected to unbalance loads especially when the pump is operating at several hundred psi. The unbalanced load is due to the discharge pressure which exerts a radial load on only a portion of the rotor (the portion that is travelling through the discharge or outlet region of the pump) as opposed to a uniform force across the entire rotor. At high pressures, it has been found that the drive shaft to which the rotor is connected, is subjected to bending loads (due to the discharge pressure generated force) sufficient to produce radial movement in the rotor. As a result, the clearance between the outer gear and the housing must be relatively high in order to accommodate the expected deflection in the shaft at high operating pressures. Alternately, the pump must be operated at reduced pressures.
In most conventional transfer gear pumps of this type, the maximum operating pressure that the pump can operate at is limited by the load that can be supported by the shaft bearing. In this type of pump, the fluid being pumped exerts an axial force on the outer rotor that is transferred to the drive shaft. Loads on the drive shaft are in turn transmitted to the shaft bearing. In conventional pumps, in order to accommodate higher pumping pressures, shaft bearings have to be increased in sized or improved in order to support the increase in mechanical load caused by the higher operating pressure.